This invention relates to the use of fiber optics to produce a time standard. Electrically produced light pulses can be transmitted by an optical fiber and then reconverted to an electrical impulse upon emerging from the fiber.
Presently, time standards are produced with voltage controlled crystal oscillators (VCXO), cesium clocks or rubidium clocks.
These existing methods of producing time standards have several disadvantages. A VCXO which has a relatively low cost (typically $800 to $3,000 for a high quality unit) has a life span of about twenty years and an accuracy of about 1 part in 10.sup.10 per day. Additionally, a VCXO suffers severe aging over its liftime which is directly related to the quality of the crystal used in the VCXO.
A rubidium clock has a high degree of accuracy in the order of 1 part in 10.sup.13 per day but its lifespan is only about ten years. Additionally, it has an initial cost of about $14,000.
A cesium clock, which is the most accurate clock available, provides an accuracy of 1 part in 10.sup.15 per day and is currently used as the primary time standard for the world. Although this clock suffers very little aging over its lifetime it will suddenly suffer extreme aging when the average five year lifespan ends. The significant cost of this clock, about $27,000, makes it unavailable for low cost systems which require high accuracy.
Cesium and rubidium clocks are both subject to local magnetic field variations and therefore they must be recalibrated against a primary time standard whenever they are moved or a change in a local magnetic field occurs.
Consequently, these clocks are not easily moved and provisions must be taken to prevent local magnetic field variations at their installation site.
Although the typical lifespan of cesium and rubidium clocks are five and ten years respectively the actual operational lifespan is much less. Both clocks utilize an immense quantity of highly complex electronics and therefore they are subject to malfunctions which are very difficult to rectify and usually require shipment of the clock to the factory for repairs. This repair time can be substantial resulting in an actual operational life much less than the clock life.